1. Field of the Invention
The present invention generally relates to a method and apparatus for image developing, and more particularly to a method and apparatus for image developing in which developer is caused to suitably form a magnetic brush in order to develop an image in a superior quality.
2. Discussion of the Background
In general, image forming apparatuses using an electrostatic recording method or an electrophotographic method, such as copiers, printers, facsimile machines, or the like, performs a common image forming operation. In such a common image forming operation, an electrostatic latent image is formed in accordance with original image information on a latent image carrying member including a photoconductive member such as a photoconductive drum, a photoconductive belt, or the like. Then, an image developing apparatus of the image forming apparatus performs an image developing operation relative to the latent image formed on the latent image carrying member so as to form a visible image.
Recently, a so-called magnetic brush image developing method using a two-component developer including toner and carriers has been mainstream in an image developing field, from viewpoints of image transferability, halftone reproducibility, stability of image development against varying temperature and humidity. With the magnetic brush image developing method, an image developing apparatus causes the two-component developer to form on a developer carrying member with a magnetic force thereof into a brush-like shape including a plurality of developer chain segments each made of chained developer particles. The developer thus formed in the brush-like shape is referred to as a magnetic brush. The magnetic brush formed on the developer carrying member supplies toner to a latent image formed on a latent image carrying member in a developing region which is formed between the developer carrying member and the latent image carrying member. The developing region is defined as a region in which the developer forms a magnetic brush on the developer carrying member and makes contact with the latent image carrying member.
The developer carrying member is generally composed of a hollow cylindrical sleeve (i.e., a developing sleeve) and a magnet member (i.e., a magnet roller) mounted inside the developing sleeve. The magnet roller forms magnetic fields for causing the developer deposited on the surface of the developing sleeve to rise in the form of a plurality of chain segments. More specifically, carrier particles contained in the developer rise along magnetic lines of force generated by the magnet roller and form developer chain segments. Onto such a developer chain segment, charged toner particles contained in the developer are deposited. The magnet roller includes a plurality of magnetic poles formed by the same plurality of magnets each of which has a rod-shape, for example. One of the magnets has a main magnetic pole (i.e., a developing magnetic pole) for especially causing the developer to form a magnetic brush relative to the developing region on the developing sleeve.
In the above configuration, when at least one of the developing sleeve and the magnet roller moves, it conveys the developer forming the rising developer chain segments towards the developing region. The developer brought to the developing region rises in the form of the magnetic brush along the magnetic lines of force generated by the main magnetic pole. As the head of the magnetic brush contacts the surface of the latent image carrying member, it yields itself. While the magnetic brush sequentially rubs against the latent image formed on the latent image carrying member at a speed determined on a basis of a difference of linear velocity between the developer carrying member and the developing sleeve, the toner is transferred from the developer carrying member to the latent image carrying member.
Conventionally, an analog image forming apparatus has been prone to cause a problem when a latent image is formed in a low contrast and has used an edge effect to compensate this problem. The edge effect is brought from relatively strong electrostatic fields which are generated around an image portion and a non-image portion of an electrostatic latent image formed on a photoconductive member. At an event that the edge effect is strong or great, or is produced, an electrostatic field is greater and an amount of toner used for image developing on an edge portion of an image is greater than that used for an inside portion of the image. As a result, the image will have a higher density. On the other hand, at an event that the edge effect is weak or small, or is not produced, an electrostatic field is smaller and an amount of toner used for image developing on an edge portion of an image is similar to that used for an inside portion of the image. As a result, the edge and inside portions of the image have even densities and the image will be produced in a superior quality.
However, a digital image forming apparatus which has recently come into widespread use has such a problem as described above. Accordingly, the digital image forming apparatus is required to develop an image in accordance with a latent image with as great a fidelity as possible so as to achieve an ideal image forming. To do this, the digital image forming apparatus is particularly required to perform a sophisticated image developing function capable of using a high image density. One known way for allowing an image developing to use a high image density is to make a developing gap narrow. The developing gap is specified as a distance between the latent image carrying member and the developer carrying member. Another known way is to make a developing nip wider. The developing nip is specified as a width of the developing region.
FIG. 1 shows variations of the edge effect when an image of one-dot-width vertical lines is developed with variations of a line density and the developing gap. In FIG. 1, the vertical axis represents a resultant edge effect ratio and the horizontal axis represents variations of the line density. When an image of one-dot-width vertical lines is developed with a line density of 150 lpi (lines per inch), the edge effect is defined as a value of 1. For example, as shown in FIG. 1, the edge effect with a developing gap (Gp) of 0.6 mm is flat at an edge effect level of 1 with the line density in a range of 150 lpi to 200 lpi but is increased to a value slightly below 1.4 with a line density of 100 lpi and to a value slightly below 1.7 with a line density of 50 lpi. For another example, the edge effect with a developing gap (Gp) of 400 xcexcm is flat at the edge effect level of 1 with the line density in a range of 120 lpi to 200 lpi but is increased to a value about 1.05 with a line density of 100 lpi and to a value slightly above 1.2 with a line density of 50 lpi. With referring also to other examples having different Gp values shown in FIG. 1 in this way, it is understood that the edge effect is prone to reduce as the developing gap is made narrower.
More specifically, it is understood that an image of one-dot-width lines with a fixed Gp is prone to receive a greater edge effect when the image has a smaller line density, or a smaller spatial frequency, that is, each line in the image is isolated. The reason is that when a line is isolated electric force lines are concentrated onto the isolated line, thereby increasing the intensity of the electric field around the isolated line by which more toner is attracted to the isolated line. As a result, the isolated line becomes thicker. It is also understood, on the contrary, that an image of one-dot-width lines with a fixed Gp is prone to receive a smaller edge effect when the image has a greater line density, or a greater spatial frequency, that is, the image is dense. The reason for this is that when a spatial frequency is great electric force lines are not concentrated onto the lines, thereby decreasing the intensity of the electric field around the lines by which less toner is attracted to the lines. As a result, each of the lines becomes thinner.
It is also understood that when the developing gap is greater the edge effect ratio is increased and that the edge effect ratio can be made closer to an ideal value of 1 by making the developing gap narrower. That is, the edge effect can be decreased when the developing gap is made narrow. When the developing gap is wider, a number of electric force lines concentrating onto an edge portion of a line in an image increases amongst the electric force lines headed towards the opposite electric pole (i.e., the developing sleeve). Accordingly, the edge portion receives more toner and, as a result, the line becomes thicker. On the other hand, when the developing gap is narrower, the developing electric field deviated aside will be headed towards the opposite electric pole and the intensity of the electric field around the edge portion will accordingly be reduced. Accordingly, an edge enhancement effect will be reduced and, as a result, a ratio of line widths according to the difference of spatial frequencies is reduced. In addition, when the developing gap is made narrower, the intensity of the developing electric filed around the gap is increased and, therefore, a developing performance will be increased.
A high developing performance also brings a high gamma development which is an advantage for the digital binary developing method. A high gamma development is known to be a way for removing granularity. In the conventional dev eloping apparatus, however, the developing nip is wider when the developing gap is made narrower. It is known that a faulty image (i.e., a rear-edge omission problem) is produced typically when a solid black image is developed with a wider developing nip. Such a rear-edge omission problem appears particularly on a rear edge portion of a solid black image or a solid half-tone image, or a rear edge portion of a cross portion of solid black lines or solid half-tone lines. Also, a development with a wider developing nip produces another faulty image in which horizontal lines are developed thicker than vertical lines in an image having horizontal and vertical lines with an equal thickness, or in which such a small image as a one-dot is not developed.
The above-described rear-edge omission problem is a phenomenon of a faulty image which appears when the latent image carrying member and the developer carrying member standing opposite each other move in the same direction in the developing region. But, when the latent image carrying member and the developer carrying member move in the opposite directions each other, the omission problem will appear on a top edge portion on an image and a phenomenon of such a faulty image is therefore referred to as a top-edge omission problem.
Referring now to FIGS. 2A and 2B, detailed behaviors of toner in the above-described mechanisms are explained. In FIG. 2A, numeral references 101 and 141 denote parts of a latent image carrying member (i.e., a photoconductive member) and a developer carrying member (i.e., a developing sleeve), respectively. A latent image carrying member 101 is usually formed in a drum-like shape but in this case it is drawn as a flat plate member for the sake of convenience. The latent image carrying member 101 and the developer carrying member have a developing gap G between the two. The latent image carrying member 101 is connected to a ground and has a latent image potential on the surface thereof. The developer carrying member 141 is connected to a certain voltage Vb connected to a ground and carries the rising chain segments at positions H1, H2, H3, and H4, for example, on the surface thereof. These positions H1-H4 are within a developing nip N.
In FIG. 2A, a potential graph corresponding to the developing nip N is additionally drawn in which a boundary between a background portion and an image portion of a latent image arrives at substantially the center of the developing nip N. The latent image carrying member 101 and the developer carrying member 141 move in the same direction, but the former moves at a speed Sp lower than a speed Ss at which the latter moves. In this sense, it is assumed that the latent image carrying member 101 is stationary relative to the developer carrying member 141. As the developer carrying member 141 moves, a rising chain segment rises at the position H1 and the head thereof starts to contact the latent image carrying member 101. The rising chain segment further moves and then passes the position H2 while a head carrier particle thereof is rubbing itself against the background portion. The rising chain segment further moves and then passes the image portion at the position H3. Subsequently, the rising chain segment falls down at the position H4 with the result that the head carrier particle is released from the latent image carrying member 101. While the head carrier particles are moving from the position H1 to the position H4, i.e., throughout the distance of the developer nip N, these particles do not change their heights and individually roll.
FIG. 10B shows another condition of the latent image carrying member 101 and the developer carrying member 141 after a certain time period. In this condition, as time passes, the latent image carrying member 101 and the developer carrying member 141 relatively move and the rear end of the latent image is brought closer to the position H4.
FIGS. 3A-3D show the behaviors of the toner particles adhered to a head carrier particle of the rising chain segment of the magnet brush, relative to a part of the latent image carrying member 101, while the rising chain segment moves from the positions H1 through to H4. FIGS. 3A, 3B, 3C, and 3D represent conditions in which the rising chain segment passes the positions H1, H2, H3, and H4, respectively.
As shown in FIG. 3A, at the position H1, toner particles T adhere to a carrier particle C in a manner relatively uniform since the position H1 locates at an inlet of the developing nip N. A shown in FIG. 3B, at the position H2, the toner particles T move away from the latent image carrying member 101 since an electric field formed by the bias Vb and the electrostatic potential of the background of the latent image carrying member 101 has an orientation in the direction from the latent image carrying member 101 towards the developer carrying member (not shown in this drawing). As a result, the number of toner particles T decreases in the vicinity of the latent image carrying member 101. More specifically, since the carrier particle C rolls while moving in the developing nip N, the surface area of the carrier particle C adjoining the latent image carrying member 101 and where the number of toner particles T decreases increases with an increase in the width of the developing nip N.
As shown in FIG. 3C, at the position H3, an electric field formed by the bias Vb and the electrostatic potential of the image portion of the latent image carrying member 101 has an orientation in the direction from the developer carrying member to the latent image carrying member 101. However, the toner particles T moving downward cannot instantaneously deposit on the image of the latent image carrying member 101. During this interval, the part of the magnet brush which has moved away from the above image portion causes toner particles Txe2x80x2 previously deposited on the latent image carrying member 101 to again deposit on the carrier particle C due to the counter charge of the particle C, as indicated by an arrow in FIG. 3C, which phenomenon is referred to as a toner return. As a result, the number of toner particles T on the carrier particle C increases while the number of toners on the trailing edge of the image formed on the latent image carrying member 101 decreases accordingly.
The counter charge of the carrier particle C decreases with the above increase in the number of toner particles T caused by the toner return phenomenon, so that the toner particles T are again caused to easily move to the head of the magnet brush. Specifically, as shown in FIG. 3D, at the position H4, the electric field directed from the developer carrying member toward the latent image carrying member 101 causes the toner particles T to move toward the latent image carrying member 101 away from the carrier particle 1. At the same time, the toner particles Txe2x80x2 returned to the carrier particle C again deposit on the latent image carrying member 101.
As shown in FIG. 2B, the trailing edge of the image portion approaches the position H4 due to the relative moment of the developer carrying member 141 and the latent image carrying member 101. A rising chain segment then falls down in the condition shown in FIG. 3C. More specifically, substantial part of the toner particles T is returned from the latent image carrying member 101 to the carrier particle C. As a result, the top carrier particle of the rising chain segment falls down with only a small number of toner particles T remaining on the image portion, ending the development. This renders the rear-edge omission problem which is particularly conspicuous when it comes to a halftone image. Moreover, when the linear velocity ratio is increased, a greater impact occurs when the magnet brush contacts the latent image carrying member 101 and reduces the adhesion of the toner particles T to the carrier particle C, thereby making the toner particles T easier to move. Under the development conditions in which the rear-edge omission problem will easily be generated, reproducibility of a horizontal line and a one-dot image will also be degraded.
In order to decrease the rear-edge omission problem, the line velocity of the developer carrying member 141 relative to the line velocity of the latent image carrying member 101 is decreased. However, this results a less toner supply and an image development with an insufficient density. Therefore, the line velocity ratio of the developer carrying member 141 relative to the line velocity of the latent image carrying member 101 is commonly set to around 1.1 to 1.2 so as to avoid an occurrence of the insufficient toner supply. The line velocity ratio of 1.2 is, however, still insufficient to develop an image in a superior quality. To improve such an image development, use of two developing carrying members has been introduced. In this case, twice of development are performed by the two developing carrying members at the line velocity of about 1.2 relative to the same latent image so that a sufficient amount of toner can be supplied. However, this solution brings problems of machine size and machine cost.
Also, as illustrated in FIGS. 3B and 3C, the carrier particle positioned on the top of the rising chain segment has a less number of toner particles thereon may negatively affect relative to the latent image by causing a toner drift for drifting a toner particle or a reduction of the counter charge. To reduce such a negative impact to the latent image, it is an efficient way to make the developing nip N narrow. On the other hand, making the developing gap G narrower is desired from the viewpoint of an increase of an intensity of the electric field promising a high development performance, so that an image development is performed with a high fidelity relative to an image which cannot expect an edge effect.
The present invention provides a novel image developing apparatus which includes a latent image carrying member and a developer carrying member. The latent image carrying member is configured to carry a latent image. The developer carrying member is provided in proximity to the latent image carrying member so as to form a developing region between the latent image carrying member and the developer carrying member and configured to carry developer which forms a magnetic brush on a surface thereof and to move the magnetic brush to the developing region so that the magnetic brush brushes a surface of the latent image carrying member in the developing region and that the latent image on the latent image carrying member is visualized. In the thus-configured novel image developing apparatus, a developing nip is formed in such a small size that a time period in which a toner of the magnetic brush moves to the developer carrying member back from the latent image carrying member is reduced when the magnetic brush brushes a non-image portion of the surface of the latent image carrying member in the developing region and a density of the magnetic brush is increased so that an electric field for image development is evenly formed.
A developing gap between the developer carrying member and the latent image carrying member may be made relatively small.
The magnetic brush may move from an upstream to a downstream of the developing region at a relatively fast speed.
The developer carrying member may includes a developing sleeve and a magnet roller which is provided inside the developing sleeve and which includes a plurality of magnets, one of which has an arrangement of a smallest half-value angle and is determined as a magnet having a developing magnetic pole.
The half-value angle of the developing magnetic pole may be about 80% of the half-value angle of adjacent magnets.
A center angle in the magnet roller between boundaries of the developing magnetic pole and a magnetic pole of one adjacent magnet and of the developing magnetic pole and a magnetic pole of another adjacent magnet may be about 60 degrees or less.
At least the developing magnetic pole amongst other magnetic poles may be formed by a rare-earth metal alloy magnet.
A magnetic force of the developing magnetic pole may be about 60 mT or more.
A developing nip formed on the developer carrying member may be greater than a diameter of a developer particle and is about 2 mm or less.
A chain segment of the magnetic brush made of the developer and formed on the developing sleeve of the developer carrying member may have a width of about 2 mm or less at a base portion thereof.
The present invention further provides an image developing apparatus which includes a latent image carrying member and a developer carrying member. The latent image carrying member is configured to carry a latent image. The developer carrying member includes a developing sleeve and a magnet roller which is provided inside the developing sleeve and has a plurality of magnets one of which has a developing magnetic pole. The developer carrying member is configured to carry a developer, to cause the developer to rise in a form of chain segments so as to form a magnetic brush on a surface of the developer carrying member with a magnetic force of the developing magnetic pole, and to cause the magnetic brush to brush a surface of the latent image carrying member so that the latent image on the latent image carrying member is visualized. In this configuration, the developing magnetic pole has in its normal direction a predetermined magnetic flux density of which attenuation rate is about 40% or more.
Further, the present invention provides a novel magnet roller for serving as an image carrying member for use in an image developing apparatus. In one embodiment, a novel magnet roller includes a developing sleeve and a magnet roller. The developing sleeve is configured to carry developer. The magnet roller is provided inside the developing sleeve and has a plurality of magnets one of which has a developing magnetic pole for causing the developer to rise in a form of chain segments so as to perform an image visualization relative to a latent image. In this configuration, the developing magnetic pole has in its normal direction a predetermined magnetic flux density of which attenuation rate is about 40% or more.
Further, the present invention provides a novel image forming apparatus which includes anyone of the image developing apparatuses described above.